It has recently been proposed to treat mined material with high intensity microwave radiation to cause formation of cracks in fragments of mined material. The fragments may include gangue and valuable material (such as copper or iron containing minerals) and the exposure of the fragments to high power-density electric fields related to the high intensity microwave radiation causes preferential heating and resultant thermal expansion of some of the components of the fragments, which results in formation of micro-cracks and macro-cracks. Such cracks improve for example energy required to break the fragments apart and improve access for leach solutions. The formation of the cracks is directly related to the value and rate of development of a temperature differential that is created during the application of the high intensity microwave radiation.